pgsql: Provide multi-block smgrprefetch().

Provide multi-block smgrprefetch().

Previously smgrprefetch() could issue POSIX_FADV_WILLNEED advice for a
single block at a time. Add an nblocks argument so that we can do the
same for a range of blocks. This usually produces a single system call,
but might need to loop if it crosses a segment boundary. Initially it
is only called with nblocks == 1, but proposed patches will make wider
calls.

Reviewed-by: Heikki Linnakangas <hlinnaka(at)iki(dot)fi> (earlier version)
Discussion: https://postgr.es/m/CA+hUKGJkOiOCa+mag4BF+zHo7qo=o9CFheB8=g6uT5TUm2gkvA@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/b485ad7f07c80efbfd47329f138f0fe3a5acf013

Modified Files
--------------
src/backend/storage/buffer/bufmgr.c | 2 +-
src/backend/storage/buffer/localbuf.c | 2 +-
src/backend/storage/smgr/md.c | 37 +++++++++++++++++++++++++----------
src/backend/storage/smgr/smgr.c | 7 ++++---
src/include/storage/md.h | 2 +-
src/include/storage/smgr.h | 2 +-
6 files changed, 35 insertions(+), 17 deletions(-)